1. Field of the Invention
The present invention relates to a multimeter having an erroneous input prevention mechanism including a shutter board. More particularly, the present invention relates to a driving mechanism for a shutter board. The present invention also relates to a positional structure of an input terminal in which a test lead is plugged into a multimeter having an erroneous input prevention mechanism.
2. Description of Related Art
In a multimeter capable of measuring voltage and electric current, a negative common terminal hole and a positive input terminal hole are provided in addition to a separate input terminal hole: for measuring resistance values. In such a multimeter, a test lead pin plug is plugged into the appropriate input terminal hole corresponding to measurement mode and range selected based on switching of the rotary switch. However, if measurement is taken with the pin plug plugged into the input terminal hole for measurement of a small current while the rotary switch is set for a large current range, an internal circuit of the multimeter may be damaged.
Japanese Laid-Open Patent Publication Hei 4-233473, the subject matter of which is incorporated herein by reference, provides a multimeter having an erroneous input prevention mechanism. In this multimeter as shown in FIG. 20, a shutter board 105 is provided with a hole 104 formed for the first input terminal hole 101, the second input terminal hole 102 and the third input terminal hole 103. A stud 107 is also provided between the shutter board 105 and the rotary switch 106. The stud 107 rotates with the rotary switch 106 as one unit and engages with the fork-shaped teeth 108 of the shutter board 105 so that each time the rotary switch 106 rotates, the shutter board 105 also rotates. This causes the hole 104 to match one of the first, second and third input terminal holes 101, 102, 103. Hence, if the second input terminal hole 102 is in an open state corresponding to the setting of rotary switch 106, the first and third input terminal holes 101, 103 are in an interrupted state. This makes it impossible for the pin plug of a test lead to be accidentally plugged into holes other than the second input terminal hole 102. The negative common terminal hole 109 is positioned outside of the range of rotation (dotted line L10) of the shutter board 105.
However, the multimeter having an erroneous input prevention mechanism 100 described above with respect to FIG. 20 has the several problems.
In many cases a multimeter is capable of coping with more than one measurement mode using one input terminal hole. Hence, even in a conventional multimeter 100, the first input terminal hole 101 is left in the open state once the stud 107 disengages from the fork-shaped teeth 108 of the shutter board 105 in the direction specified by the arrow T because the shutter board 105 does not rotate any further. However, the conventional multimeter 100 is designed to utilize disengagement of the stud 107 from the fork-shaped teeth 108. Hence, a problem of little design freedom arises such that only the first and third input terminal holes 101, 103 can be assigned more than one mode while the second input terminal hole 102 cannot be assigned plural modes.
Moreover, the multimeter 100 described in FIG. 20 always leaves the negative common terminal hole 109 in the open state by forming the negative common terminal hole 109 at a location far away from the rotational range of the shutter board 105. Hence, there is little design freedom in terms of size and range of rotation of the shutter board 105. This creates a problem in that the multimeter 100 cannot be miniaturized due to the accommodation of the negative common terminal hole 109 located far away from the range of rotation of the shutter board 105.
Moreover, the multimeter 100 described in FIG. 20 is structured such that the input terminal hole 109 is selected to be in the open state only by the hole 104 of the shutter board 105. This causes each input terminal hole to be arranged on the locus of the hole, which is a restriction. Hence, there is little freedom in changing the position of the input terminal hole.